The lymphatic system is a part of the immune system and protects the body against infection and invasion by foreign organisms. The lymphatic system consists of lymphatic vessels and lymphatic tissue. Lymphatic vessels are a network of thin tubes that branch, like blood vessels, into tissues through the body. In mammals, most tissues and organs are drained by the lymphatic system. Lymphoid tissue is found in many organs, particularly the lymph nodes. Lymphoid tissues contain lymphocytes, but they also contain other types of immune cells such as macrophages, dendritic cells and eosinophils. Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against infections and spread of tumors.
The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation associated with the tissue. The central or primary lymphoid organs such as bone marrow generate lymphocytes from immature progenitor cells. Secondary or peripheral lymphoid organs maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen. Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT). Tonsils are collections of lymphoid tissue facing into the aerodigestive tract, i.e. the oropharynx and the nasopharynx. The set of lymphatic tissue known as Waldeyer's tonsillar ring includes the adenoid tonsil, two tubal tonsils, two palatine tonsils, and the lingual tonsil. When used unqualified, the term “tonsils” most commonly refers specifically to the palatine tonsils, which are masses of lymphatic material situated at either side at the back of the human throat. “Tonsils” as used herein refers to tonsils within the Waldeyer's tonsillar ring and includes pharyngeal tonsils, tubal tonsils, palatine tonsils and/or lingual tonsils.
Pathogenic organisms such as Staphylococci, Streptococci, Haemophilus are common organisms in all animals including people. Streptococcus suis (S. suis), for example, is a gram-positive bacteria. It is an increasingly important pathogen affecting the swine industry worldwide and can reside as a commensal in the upper respiratory tract of pigs. S. suis can be shed from sows through bodily fluids such as vaginal secretions There are many serotypes of S. suis and they can vary in their pathogenicity. Different types S. suis predominate in different countries with type 2 being the most common. Newborn piglets may become colonized with S. suis via vertical transmission during farrowing and suckling. Previous studies have found that S. suis is frequently isolated from the tonsillar area of piglets before and after weaning. Infectious S. suis can spread to other animals during the production stage, or cause disease due to stress at weaning.
Decreased performance and mortality resulting from S. suis infection have a significant economic impact on swine production. Streptococcal infections are usually characterized by variety of conditions including meningitis, septicemia, polyserisitis, arthritis, bronchopneumonia and endocarditis. The incidence of meningitis and other conditions related to streptococcal infections may be prevented by decreasing stocking density, minimizing mixing and improving ventilation and temperature control. Currently, there are no vaccines against S. suis infections. If disease is suspected or identified, medications such as phenoxymethyl penicillin, tetracyclines, synthetic penicillins or other antibiotics may be administered. These methods are expensive and widespread use of these can lead to resistant strains. The timing of administration of these medications can also be critical and are generally applied only when the disease levels on the farm are high. Attempts to eradicate the tonsillar carrier state of S. suis in early-weaned pigs with a number of antimicrobials have not been successful. Moreover, new serotypes are being isolated each year.
There is growing concern about the impact of S. suis as a zoonotic agent, particularly for individuals who handle pigs and pig carcasses. Southeast Asian countries such as China, Thailand and Vietnam have experienced the majority of human cases. A recent US study reported that individuals exposed to swine had higher titers of antibodies to serotype 2 S. suis, compared with non-exposed individuals.
Periodontal diseases characterized by infection with gram-negative pathogens such as Porphyromonas gingivalis can induce a chronic inflammatory response. This can result in oral inflammatory bone destruction. Recent evidence suggests a role for P. gingivalis mediated periodontal disease as a risk factor for several systemic diseases including diabetes, stroke and atherosclerotic cardiovascular disease. Infections of the oral cavity by microorganisms such as Candida Albicans can occur in humans and are often side effects in individuals being treated for other conditions. Infection of the mouth and throat with C. Albicans in conditions such as thrush can make routine eating and swallowing extremely uncomfortable.